This invention relates to the anodization of aluminum electrolytic capacitor foil at low current density to obtain a barrier layer oxide with a high degree of crystallinity and increased capacitance. More particularly it relates to a process carried out at an average current density of 20 .mu.A/cm.sup.2 to 3 mA/cm.sup.2 of effective surface area in a aqueous solution of a C.sub.4 to C.sub.7 unsubstituted aliphatic dicarboxylic acid containing 0 to 0.05 wt. % of citric or tartaric acid followed by one or more relaxation and reanodization treatments.
Crystalline barrier layer oxide can be produced by anodizing aluminum in citric, tartaric or boric acid anodizing electrolytes. Unfortunately, citric acid and tartaric acid are both fairly aggressive acids and will start dissolving the oxide formed. Both electrolytes are relatively inefficient anodizing electrolytes and have a slow formation rate, e.g. less than 10 V/min at a current density of 1 mA/cm.sup.2. This results in a longer anodization time and increased power consumption.
Other organic acids, particularly unsubstituted aliphatic dicarboxylic acids, are more efficient anodization electrolytes, but heretofore they have not been known to produce any significant amount of crystalline oxide. The amount of crystalline oxide is associated with higher capacitance and also with oxide instability unless the oxide is properly relaxed and reanodized. Amorphous oxide is stable without relaxation and reanodization, but it is of a lower capacitance.
It would be desirable to be able to form a barrier layer oxide with increased crystallinity in an efficient anodization electrolyte and to be able to stabilize the oxide so it can sustain the formation voltage.